An image forming system includes an enclosure supply device including an enclosure container, a first image forming unit to form an image on the sheet, an envelope supply device including an envelope container, an input device to input an insertion job, an insertion device to insert into the envelope at least one sheet on which the first image forming unit has formed the image and including a temporary storage tray to temporarily store the sheets inserted into the envelope as well as an envelope retainer to hold the envelope at an insertion position where the sheets are inserted into the envelope, and a controller to cause the second image forming unit to complete image formation on the envelope and conveyance of the envelope to the insertion position before a last sheet of the multiple sheets inserted into the single envelope reaches the temporary storage tray.